An experimental model of mammary carcinoma was developed in the rat and characterized by the ability to produce diffuse metastasis in 95% of subjects within 72 hr from transplant. This model permitted preparation of animals with clinically silent metastases as observed in women undergoing a mastectomy for mammary carcinoma. The model was utilized to study the influence of pregnancy and lactation on the growth rate of clinically silent metastases at the time of conception. The results showed that pregnancy prolongs the survival time of rats bearing metastases of this mammary carcinoma. For the first time, to our knowledge, support has been obtained under experimentally controlled conditions of sporadic clinical observations suggesting that pregnancy in women of child bearing age operated on for breast cancer need not necessarily be avoided, providing it occurs after the treatment for the primary breast cancer has been completed and lactation following delivery is not permitted.